Implement utf8 standard library #253

Problem

The utf8 standard library is not implemented. utf8.lua from the official suite is deferred entirely on local utf8 = require'utf8' (yields true from our require cache rather than the library table).

Scope

Lua 5.3 §6.5 — six surface functions plus one field:

• utf8.char(...) — codepoints → UTF-8 string
• utf8.codepoint(s [, i [, j]]) — UTF-8 string → codepoints
• utf8.codes(s) — stateless iterator over (byte_pos, codepoint) pairs
• utf8.len(s [, i [, j]]) — number of codepoints (or nil, byte_pos on invalid)
• utf8.offset(s, n [, i]) — byte position of the n-th codepoint
• utf8.charpattern — string constant: pattern matching one valid UTF-8 byte sequence

All operations are over bytes; Elixir's String.next_codepoint/1 handles validation for us.

Suggested approach

1. New module lib/lua/vm/stdlib/utf8.ex implementing Lua.VM.Stdlib.Library. Follow the layout of lib/lua/vm/stdlib/math.ex.
2. Register via install_library/2 in lib/lua/vm/stdlib.ex so require"utf8" resolves it from package.loaded.
3. New test file test/lua/vm/utf8_test.exs covering the well-defined cases plus invalid-sequence handling.

Acceptance

• mix test passes.
• utf8.lua either passes outright or progresses to a well-defined later failure that we can triage separately. Remove its :all entry from test/lua53_skips.exs.